Environmental tobacco smoke (ETS), also known as secondhand smoke or passive smoking, is a threat to non-smokers' health, causing and exacerbating asthma, as well as upper and lower respiratory infections, cardiovascular disease (CVD), and lung cancer. There is no safe level of ETS exposure. Human ETS exposure is highest among low-income groups and non-smoking households in multiunit housing (MUH), including public housing, have elevated levels of ETS. Public housing residents' vulnerability to ETS exposure is heightened because they are in housing of last resort without the means to avoid exposure by changing residences. Public policies such as workplace smoking bans have resulted in demonstrated improvements in population health, leading to lower levels of ETS exposure for non-smokers and lower rates of adverse cardiovascular and respiratory outcomes. In June of 2009, the Department of Housing and Urban Development called on local housing authorities to implement smoke-free policies. Because MUH is one of the last frontiers of smoking regulation, it is not clear whether smoke-free housing rules will be effective at reducing non-smokers' exposure to ETS, particularly in public housing, and to date there have been no comprehensive studies. In 2010, the city of Boston, MA declared that the properties of the Boston Housing Authority (BHA), including residences, will become entirely smoke-free in the fall of 2012, making Boston's the largest public housing authority in the country with such plans. Our proposed study seizes on this unique moment in time, using a quasi- experimental, longitudinal design to test the hypothesis that smoke-free policies in public housing reduce residents' exposures to ETS. The BHA is our intervention site. Our control site is the Cambridge Housing Authority (CHA) in Cambridge, MA where no smoke-free housing policy will be implemented. We will conduct difference-in-differences analyses to test whether changes in ETS levels and exposure from summer 2012 (pre-BHA smoke-free housing policy) to summer 2013 (post-BHA smoke-free policy) are greater for the intervention site (BHA) than for the control site (CHA). At least 150 randomly selected non-smoking households from each site will undergo pre and post assessments, and one resident from each household will be included in the study. ETS exposure will be assessed by measuring participants' saliva cotinine; air quality in households and common areas (airborne nicotine and particulate matter - PM2.5); and participants' self- reports. In addition, the sources of exposure to ETS will be determined by airborne nicotine, air flow, and PM2.5 measurements, as well as residents' self-report. Additionally, residents will be surveyed on their knowledge, attitudes, beliefs, and behaviors towards ETS and smoke-free policies. Understanding Boston's experience implementing the policy, and understanding the impact the policy's effect on public health will provide crucial information to policy makers and other housing authorities as these policies become more commonplace.